Infrared intrusion detection apparatus

ABSTRACT

The infrared system employs a multiple mirror optical system for focusing and collecting radiation from a moving object such as an intruder and directing the energy to a sensing element. In a preferred arrangement the optical system comprises a convex mirror, a planar array of plane mirrors and a concave or parabolic collector having the sensing element disposed at the focal point thereof. The sensing element couples to a detection circuit which includes modulating means, an impedance matching circuit, amplifier circuit, filtering circuit and trigger circuit. In an alternate embodiment the optical system includes a series of plane mirrors disposed along an arcuate locus, and a concave or parabolic reflector for collecting parallel rays from the plane mirrors. In still another embodiment, a series of spaced spherical mirror segments are used having varying focal lengths.

United States Patent 11 1 Reiss et al.

1 1 INFRARED INTRUSION DETECTION APPARATUS [75] Inventors: Martin H. Reiss, Newton; Elias E.

Solomon, Duxbury, both of Mass.

[52] US. Cl. 250/338; 250/353 [51] Int. Cl. GOlt 1/16 [58] Field of Search 250/338, 340, 342, 347,

[56] References Cited UNITED STATES PATENTS 9/1971 Beach .1 250/347 11/1972 Berman 340/258 D TO CENTER OF CURV.

Primary Examiner-James W. Lawrence Assistant Examiner-Davis L. Willis Attorney, Agent, or Firm-Wolf, Greenfield & Sacks [57] ABSTRACT The infrared system employs a multiple mirror optical system for focusing and collecting radiation from a moving object such as an intruder and directing the energy to a sensing element. In a preferred arrangement the optical system comprises a convex mirror, a planar array of plane mirrors and a concave or parabolic collector having the sensing element disposed at the focal point thereof. The sensing element couples to a detection circuit which includes modulating means, an impedance matching circuit, amplifier circuit, filtering circuit and trigger circuit. In an alternate embodiment the optical system includes a series of plane mirrors disposed along an arcuate locus, and a concave or parabolic reflector for collecting parallel rays from the plane mirrors. in still another embodiment, a series of spaced spherical mirror segments are used having varying focal lengths.

14 Claims, 9 Drawing Figures OBJECT PRINClPALj AXIS Pmmmwzv 1% 11886360 SHEET 1 PRIOR ART FIG 1 FIG 3 4 INFRARED INTRUSION DETECTION APPARATUS FIELD OF THE INVENTION The present invention relates in general to an infrared intrusion detection system, and pertains more particularly, to an infrared system for detecting an intruders motion and having an improved optical collecting system and improved detection and supervision circuitry.

BACKGROUND OF THE INVENTION A typical prior art patent is U.S. Pat. No. 3,703,7l8 which discloses an optical collecting means for use in an infrared intrusion detector system. FIG. 1 of the present invention discloses a segmented spherical mirror system of the type disclosed in that patent.

Some of the previous problems associated with optical systems using detection of radiated energy, whether visible or invisible, stem from a physical limitation; that to achieve a sharp focus the mirror or lens system has to be of a relatively small size. Accordingly, known optical systems have a correspondingly small collecting area and generally only a small portion of the image can be utilized by a typical sensing element.

As an example of a system of the type disclosed in U.S. Pat. No. 3,703,718, consider an object, such as a person, having a length of two meters positioned at a distance of 10 meters from the detector. Assuming ideal focusing, the image is at or very near the focal point of the mirror. The image length is represented by the following equation:

r= U (x Thus, in the above example the image has a length of approximately two centimeters. However, a typical sensor covers a length of only 0.2 centimeters and thus only about one-tenth of the image is available for generating a signal at the detector. Larger sensors, can be constructed, but are impractical as they become excessively costly.

Another problem associated with using an optical system ofthe type disclosed in US. Pat. No. 3,703,718, is concerned with the orientation of the image with respect to the detector. The detectors are normally mounted in an enclosure as taught by that patent with a suitable window. As a result, and particularly because of the concave mirror system, not all of the radiation reaches the sensing element and thus the efficiency of the system is impaired.

However, in accordance with the present invention, primarily only parallel rays are collected and thus the image is sharply focused.

Other problems relating to these prior art infrared detection systems pertain in particular to the detection circuitry. Difficulties arise when it is attempted to detect low level signals using detectors which have a comparatively high noise level. The sensors themselves are usually high impedance devices such as a thermistor. Impedance matching networks are therefore required and because of the low-level signal high gain amplifiers are also required. There is an attendant tendency in these circuits for a low frequency component to be present which at the output. constitutes a not negligible portion of the signal level. A typical system would use a filter network.

However, in accordance with the present invention and to reduce the need to filter this low-level frequency signal, and also to supervise the operation of the sens ing element, a modulation technique is used which greatly simplifies the filtering operation and level de tection of the signal.

Accordingly, one important object of the present invention is to provide an improved optical system for an infrared intrusion detective system and that is characterized by an increase in the amount of radiation that is collected.

A further object of the present invention is to an optical collecting system for gathering parallel rays from the object which are collected at a point focus.

Another object of the present invention is to provide an optical system in accordance with the preceding ob ject and that is relatively simple in construction, easily constructed, readily adjustable, and can be manufactured at reasonable cost.

Another important object of the present invention is to provide improved detection and supervision circuitry for use in an infrared detection system.

SUMMARY OF THE INVENTION In accordance with the present invention there is provided an optical system or apparatus for use in an infrared intrusion detection system and generally comprising a sensing element for receiving infrared radiation and generating an electrical signal corresponding thereto, a convex reflector surface, an array of plane mirrors and a collector reflector. The convex reflector surface receives the infrared radiation from objects in a field of view and has a defined principle axis. The rays reflected from the convex surface are parallel to this principle axis. The array of plane mirrors, which is formed as a one piece structure, is arranged in a single plane in front of the convex reflector surface. The plane defined by these plane mirrors is disposed nonorthogonally to the principal axis of the convex reflective surface. The collector reflector is for receiving radiation reflected from the plane mirrors and directing this radiation toward the sensing element. The array of plane mirrors in one embodiment comprised a 3 X 3 array for sectioning the field of view into essentially nine discreet spatial viewing areas.

In one embodiment the plane mirrors are of square configuration and in another embodiment they are of triangular configuration. In another optical arrange ment diclosed herein, there is provided a series of plane mirrors disposed along an arcuate locus for directing radiated energy to a parabolic or concave collector having the sensing element disposed at the focus thereof. In still another arrangement a typical segmented spherical mirror is used but the sensing element is disposed asymmetrically so as to receive more radiation from one of the mirror segments collecting the radiation from the furthest point of detection in the field of view. In still a further arrangement, a series of spaced spherical mirrors are employed having varying focal lengths for providing improved collection of the radiated energy.

In accordance with the present invention there is also provided improved detection and supervisory circuitry. In accordance with this invention there is provided a relatively low frequency, for example around l hertz, signal which is coupled in series with the sensing element. The modulated signal is coupled by way of an impedance matching circuit and an amplifier circuit to a low frequency filter transformer circuit which is for removing low frequency noise below the modulating frequency. A trigger circuit is provided and is activated when the modulating signal disappears which in turn is caused by the detection of an intruder by the sensor. With this modulating technique the failure of a sensor is also immediately detected.

BRIEF DESCRIPTION OF THE DRAWINGS Numerous other objects, features and advantages of the invention will now become apparent upon a reading of the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a prior art optical collection apparatus for an infrared intrusion detection system;

FIG. 2 is a partially cross-sectional view of an optical mirror system constructed in accordance with the principles of the present invention;

FIG. 3 is an auxiliary elevational view of the plane mirror array shown in FIG. 2',

FIG. 4 is an alternate embodiment for the plane mirror array shown in FIG. 3;

FIG. 5 shows another embodiment of an optical collecting system of the present invention;

FIG. 6 shows still another optical system of the present invention;

FIG. 7 shows still a further optical system of the present invention;

FIG. 8 is a partially block and partially circuit diagram of the detection circuit of the present invention; and

FIG. 9 is a detailed circuit diagram corresponding to the diagram shown in FIG. 8.

DETAILED DESCRIPTION As previously indicated, FIG. I shows a prior art opti cal collecting system for an infrared intrusion detection system, and of the type disclosed in US. Pat. No. 3,703,718. This system comprises a sensing element 10 and an optical means 12 which includes a plurality of spaced reflective members I4. This arrangement provides a number of spaced apart sector-shaped fields of view corresponding to the number of reflective members employed. When an intruder enters the secured area. each time he passes into or out of one of the dis creet fields. the level of radiation in that field changes suddenly. These sudden changes are detected at the sensing element 10 which produces a corresponding electrical signal as discussed in more detail hereinafter.

Previously, the disadvantages associated with this system have been discussed in some detail.

An improved optical system in accordance with the present invention is shown in FIG. 2. This system generally comprises a convex mirror I6, a plane mirror array 20, and a parabolic or concave collector mirror 24. A conventional sensing element 26 is disposed at the focal point of mirror 24. This sensing element may be a thermistor, thermopile, or a pyro-electric sensor.

The convex mirror 16 may be of the type having a sil vered front or could he a stainless steel reflective mir ror.

FIG. 2 also shows diagramatically an object 22 in three different positions at different distances from the optical system. The rays 28 emanating from the object are reflected from the reflective surface of mirror 16 toward plane mirror array 20. In FIG. 2 the plane mirror arrays is shown as extending along the direction of plane Pl. FIG. 2 also shows the principle axis Al associated with convex reflector 16. The plane PI is disposed non-orthorgonally to the principal axis Al.

Referring also now to FIG. 3, the plane mirror array is shown as comprising a matrix of rectangular or square reflective surfaces identified in FIG. 3 as surfaces 31A, 32A, 33A; 31B, 32B, 33B; and 31C, 32C, and 33C.

The areas intermediate these reflective surfaces are masked with a non-reflective material such as a non reflective tape, and the areas 35 and 36 are preferably substantially open to permit passage through the array of incoming radiation from the object.

The rays 28A reflected from convex mirror 16 are parallel to the principal axis Al and these rays are reflected from the reflective surfaces of the mirror array 20. The rays intercepted by the plane mirror array are reflected as rays 288 to the reflector surface of concave mirror 24. These rays are directed by mirror 24 to the focal point where sensing element 26 is disposed.

The arrangement shown in FIG. 2 essentially sections the field of view into nine discreet sector fields. Considering object 22 at position I, and assuming that the ob ject is positioned so that radiation is reflected from surface 32A, then a detection occurs at sensing element 26. As the object moves from position 1 to position 2 the sector field is left and at position 2 radiation through window 35 is directed to reflector surface 328 and a subsequent detection occurs at sensing element 26. Similarly, at position 3 the radiation is reflected from surface 32C and a detection also occurs then.

In FIG. 3 the movement in the direction 1-2-3 corre sponds to tangential movement relative to the optical system causing sequential detection as the movement occurs. For example, tangential movement may be detected in sequence by reflections from surfaces 31B, 32B and 33B. Sequential detections also may occur by departing and re-entering the same sector field.

FIG. 4 shows an alternate arrangement that can be substituted for the array of FIG. 3. In FIG. 4 there are shown nine reflective surfaces identified by the reference character 35. This arrangement ensures maximum change in energy when an intruder moves from the apex of the field of view of one mirror to the base of the field of view of the next mirror and vice versa.

The system shown in FIG. 2 thus collects divergent rays from the object to produce parallel rays 28A which are incident onto the plane mirror array 20 which is inclined at an angle to the principle axis A1 of the convex mirror l6. The parallel rays are then collected either by a concave or parabolic mirror and focused onto the detector 26. This system has a much larger field of view than the prior art systems and is capable of collecting much more radiation from an object. For example, an object having a length of two meters and disposed at 30 meters from the optical system has a one-third meter length in the field of view of the convex mirror. This is over 16 times more than is viewed by a concave mirror where the parallel rays are all brought to a point focus. However, because only convergent rays 28 produce parallel rays 28A this improvement in energy collection is somewhat reduced.

FIG. 5 shows another optical arrangement of the present invention which is somewhat more simplified than the arrangement shown in FIG. 2. This system comprises an arcuate reflector member 40 which essentially replaces the convex mirror 16 and plane mirror array shown in FIG. 2. The system of FIG. 5 also includes a parabolic or concave reflector 42 similar to the reflector 24 as shown in FIG. 2. A sensing element 44 is disposed at the focal point of reflector 42. The member 40 comprises a series of adjacent plane mirrors 46A, 46B, 46C. 46D, 46E, and 46F. The plane mirrors are disposed along an arcuate locus 49 in a manner so that all of the rays 50 reflected from these plane mirrors are directed in parallel to parabolic col lector 42. Each mirror is disposed at an angle to its adjacent mirror as indicated in FIG. 5.

In FIG. 5 the object 48 is shown in three different positions and the rays emanating from the object are shown being reflected from respective plane mirror surfaces 46A, 46B, and 46F. As the object moves be tween the positions shown in FIG. 5 a chopping action is provided by the window space between each of the plane mirror surfaces.

FIG. 6 shows still another optical system of the present invention employing spherical or concave mirror segments 50, 51, and 52 which are disposed along a spherical locus 54. This arrangement structurally appears similar to the prior art arrangement shown in FIG. 1. However, the sensing element 56 is not disposed symmetrically as shown in FIG. 1 but is disposed asymmetrically closest to or at the focal point of mirror segment 50. The mirror 50 is disposed for receiving radiation from object 58 at the remote position 1. Because it is more difficult to detect radiation at a distance the sensor 56 is disposed more closely to reflective surface 50 so as to receive more radiation from distant objects and lesser radiation from closer objects wherein the attenuation of the radiation is less.

FIG. 7 shows still another optical arrangement of the present invention including spherical mirror segments 60A, 60B. 60C, 60D, and 60E arranged similarly to the mirror shown in FIG. 6. The sensing element 62 is disposed symmetrically in FIG. 7. However, the focal lengths of each of the mirror segments is different. The mirror segment 60C has the shortest focal length, mirrors 60B and 60D have somewhat longer focal lengths and mirrors 60A and 60E have still longer focal lengths. The mirror 60C is disposed at a first locus 63, the mirrors 60B and 60D are disposed at a second locus 64 and the outer mirror segments 60A and 60E are disposed at a locus 65.

FIG. 8 is a diagram partially in block form ofa detection circuit in accordance with the present invention and which couples to a sensor 70. The sensor 70 is disclosed in FIG. 8 as a thermistor which has one side coupling through resistor 71 to ground and the other side coupling to bias element 72 which may be a resistor or another thermistor. Element 72 also couples to a supply V which is a stabilized regulated power supply. A modulating signal is coupled by way of input terminal 73 to the junction between resistor 71 and sensor 70. In one embodiment this modulating signal was a 15 hertz sinusoidal or squarewave signal.

The output of sensor couples to matching circuit 74 and from there to amplifier 75. The output of amplifier 75 couples to a filter 76 which is a low frequency blocking filter for removing low frequency components in the frequency spectrum of one-half to 2 hertz. The output of filter 76 couples to a summing or integrating circuit 77 and from there to a trigger circuit 78 which preferably includes an alarm relay and associated audible or visible alarm means.

When sufficient energy is received by sensor 70, the amplifier 75 becomes saturated and the modulating frequency disappears or is shunted. When this signal is removed at the output of the amplifier the trigger circuit 78 is activated and an alarm condition prevails. Alternatively, when the sensor is not receiving energy the trigger circuit 78 is prevented from operating the alarm.

It can also be seen from the diagram of FIG. 8 that should the sensor 70 become a short circuit. for example, or an open circuit, the amplifier 75 saturates and an alarm condition exists. In this way, supervision of the circuit is readily obtained with this modulation technique.

One important features of the present invention is the use of a thermistor for element 72. This thermistor is preferably selected to be similar in its characteristics to the sensor 70 and thus the element 72 functions as a temperature compensation element for maintaining a constant threshold voltage at the output of the sensor regardless of temperature fluctuations.

For a more thorough understanding of the operation of the diagram shown in FIG. 8, reference is now made to a complete circuit diagram which is shown in FIG. 9. In FIG. 9 the modulating frequency coupled to input terminal 73 is from modulator circuit 80 which is sub stantially a conventional relaxation oscillator circuit and generally includes transistor 81, timing capacitor 82, potentiometer 83, and associated biasing resistors. The components of the circuit are preselected so that the modulator circuit 80 operates at a frequency of, for example. l5 hertz.

FIG. 9 also shows the sensor 70, resistor 71 and bias element 72 arranged in the same manner shown in FIG. 8. The sensor 70 and element 72 essentially comprise a voltage divider wherein the output voltage coupled to impedance matching circuit 74 is a function of the resistance of sensor 70 as controlled by the received radiation.

The impedance matching network 74 comprises basically an operational amplifier and associated circuitry. The amplifier 85 has an extremely high input impedance and thus provides an impedance interface between the sensor 70 and the remainder of the detection circuitry. The output of operational amplifier 85 couples to a high frequency blocking circuit 86 which includes a bridged-T filter network 87 which is of con ventional design. This filter network couples to a transistor 88 and the emitter of the transistor couples by way of capacitor 89 to the first stage of amplifier 75. This first stage includes an operational amplifier 90 which couples to the second stage which includes a second operational amplifier 91. The filter circuit 86 is for blocking any high frequency signals typically in the range of 120 hertz or possibly higher in frequency.

The circuitry discussed to this point is designed so that when the sensor is not detecting any radiation, the modulating signal at the input terminal 73 is passed unaffected to the output of operational amplifier 91. If the sensor 70 does detect radiation from a body the voltage coupled to amplifier changes suddenly and the signal saturates amplifiers and 91 thereby block ing the modulating signal to filter circuit 76.

The filter circuit 76 comprises a transformer 92 having a primary and secondary winding. The primary winding couples from the output of amplifier 75 and the secondary winding couples to integrator circuit 77. Transformer 92 functions as a low frequency blocking filter and also functions as a network for passing the modulating signal to the intergrating circuit 77.

The integrating circuit 77 generally comprises transistor 93 diode 94 and integrating capacitor 95. when the modulating signal is present across the secondary of transformer 92 transistor 93 is periodically conducting and a charge path is provided by way of diode 94 to capacitor 95. When capacitor 95 is charged sufficiently this voltage is coupled to transistor 96 of trigger circuit 78. Transistor 96 is maintained in conduction and therefore transistor 97 is cut-off. Under this condition, the output alarm transistor 98 is also maintained cut-off and the alarm relay 99 is not energized.

Alternatively. if the modulating frequency is removed when a detection occurs transistor 93 ceases conduction, capacitor 95 discharges. transistor 96 turns off. and transistors 97 and 98 conduct causing energization of relay 99 thereby indicating an alarm condition. Capacitor 100 of the output circuit is a form of filter capacitor for providing some amount of time delay to prevent erroneous triggering.

In another embodiment. the alarm relay may be normally energized and is de-energized on receipt of an alarm signal. Having described a limited number of embodiments of the present invention it should now become apparent that other embodiments and modifications thereof should fall within the spirit and scope of the present invention. For example, in FIG. 2 there is shown a preferred optical system of the present invention wherein the array of plane mirrors includes the masking for providing the chopping or modulating action. In an alternate embodiment the convex mirror could be provided with a non-reflective grid pattern instead of the planar array. in that case the planar array would be replaced by a single series of plane reflector surfaces. The series of plane mirrors extends in one direction covering one direction of motion and the convex mirror segments extend in the orthogonal direction covering an orthogonal direction of motion.

What is claimed is:

1. Optical apparatus for an infrared intrusion detection system comprising;

a sensing element for providing an electrical signal corresponding to the level of infrared radiation impinging on the element,

a convex reflective surface for receiving the radiation from objects in the field of view and reflecting at least some of the radiation in rays parallel to the principle axis of the convex reflective surface,

an array of spaced reflective surfaces disposed to receive the rays from said convex surface,

and a collector reflector for receiving radiation re flected from at least one of said reflective surfaces of said array and directing said radiation to said sensing element.

2. The apparatus of claim I wherein said convex reflective surface has a principle axis and said array of reflective surface are disposed in a plane, said axis being non-orthogonal to said plane.

3. The apparatus of claim 2 wherein said array includes means commonly mounting said spaced reflective surfaces.

4. The apparatus of claim 3 wherein said spaced reflective surfaces are disposed in a plurality of sets spaced to permit radiation from the field of view to pass to the convex reflective surface.

S. The apparatus of claim 4 wherein the reflective surfaces of each set are separated by a non-reflective member.

6. The apparatus of claim 1 wherein each said reflective surface of said array is of square or rectangular shape.

7. The apparatus of claim 1 wherein each said reflective surface of said array is of triangular shape.

8. The apparatus of claim 1 wherein said collector re flector is a concave reflector having the sensing ele' ment disposed at the focal point thereof.

9. Optical apparatus for an infrared intrusion detection system to detect the motion of objects or persons in a field of view that is to be secured comprising;

a sensing element for providing an electrical signal corresponding to the level of infrared radiation impinging on the element,

a series of stationary adjacent plane mirrors disposed along an arcuate locus to form a concave reflective surface having a window space between each adjacent mirror. each said mirror angled only slightly from its adjacent mirror, each said mirror for receiving radiation from objects in the field of view and reflecting at least some of the radiation in rays in parallel,

a collector reflector for receiving the parallel rays reflected from at least one of said mirrors and directing primarily only the parallel rays to said sensing element.

10. Optical apparatus for an infrared intrusion detec tion system comprising;

a sensing element for providing an electrical signal corresponding to the level of infrared radiation impinging on the element,

and a plurality of spaced apart reflective surfaces for gathering infrared radiation from a plurality of discrete fields of view and focusing said radiation on said sensing element,

said sensing element being disposed closer to one of said reflective surfaces to more sharply focus radia tion received from distant objects, the other of said reflective surfaces for receiving radiation from objects located closer to said reflective surfaces than said distant objects 11. The optical apparatus of claim 10 wherein said spaced apart reflective surfaces are concave and arranged along an arcuate locus.

12. Optical apparatus for an infrared intrusion detection system comprising;

a sensing element for providing an electrical signal corresponding to the level ofinfrared radiation im pinging on the element,

and a plurality of spaced apart reflective surfaces for gathering infrared radiation from a plurality of discrete fields of view and focusing said radiation on said sensing element,

at least some of said reflective surfaces having different focal lengths, adjacent reflective surfaces being of different focal length and disposed along different locii of curvature.

13. The optical apparatus of claim 12 wherein the reflective surfaces include a center surface of first focal length and adjacent surfaces of progressively larger focal length.

14. Optical apparatus for an infrared intrusion detection system comprising a sensing element for providing an electrical signal corresponding to the level of infrared radiation impinging on the element.

and a plurality of spaced apart concave reflective surfaces arranged along an arcuate locus and for gathering radiation from a plurality of spaced apart reflective surfaces for gathering infrared radiation from a plurality ofdiscrete fields of view and focus' ing said radiation on said sensing element,

said sensing element being disposed at the focal point of only one of said reflective surfaces thereby being generally closer to said one reflective surface to more sharply focus radiation received from distant 

1. Optical apparatus for an infrared intrusion detection system comprising; a sensing element for providing an electrical signal corresponding to the level of infrared radiation impinging on the element, a convex reflective surface for receiving the radiation from objects in the field of view and reflecting at least some of the radiation in rays parallel to the principle axis of the convex reflective surface, an array of spaced reflective surfaces disposed to receive the rays from said convex surface, and a collector reflector for receiving radiation reflected from at least one of said reflective surfaces of said array and directing said radiation to said sensing element.
 2. The apparatus of claim 1 wherein said convex reflective surface has a principle axis and said array of reflective surface are disposed in a plane, said axis being non-orthogonal to said plane.
 3. The apparatus of claim 2 wherein said array includes means commonly mounting said spaced reflective surfaces.
 4. The apparatus of claim 3 wherein said spaced reflective surfaces are disposed in a plurality of sets spaced to permit radiation from the field of view to pass to the convex reflective surface.
 5. The apparatus of claim 4 wherein the reflective surfaces of each set are separated by a non-reflective member.
 6. The apparatus of claim 1 wherein each said reflective surface of said array is of square or rectangular shape.
 7. The apparatus of claim 1 wherein each said reflective surface of said array is of triangular shape.
 8. The apparatus of claim 1 wherein said collector reflector is a concave reflector having the sensing element disposed at the focal point thereof.
 9. Optical apparatus for an infrared intrusion detection system to detect the motion of objects or persons in a field of view that is to be secured comprising; a sensing element for providing an electrical signal corresponding to the level of infrared radiation impinging on the element, a series of stationary adjacent plane mirrors disposed along an arcuate locus to form a concave reflective surface having a window space between each adjacent mirror, each said mirror angled only slightly from its adjacent mirror, each said mirror for receiving radiation from objects in the field of view and reflecting at least some of the radiation in rays in parallel, a collector reflector for receiving the parallel rays reflected from at least one of said mirrors and directing primarily only the parallel rays to said sensing element.
 10. Optical apparatus for an infrared intrusion detection system comprising; a sensing element for providing an electrical signal corresponding to the level of infrared radiation impinging on the element, and a plurality of spaced apart reflective surfaces for gathering infrared radiation from a plurality of discrete fields of view and focusing said radiation on said sensing element, said sensing element being disposed closer to one of said reflective surfaces to more sharply focus radiation received from distant objects, the other of said reflective surfaces for receiving radiation from objects located closer to said reflective surfaces than said distant objects.
 11. The optical apparatus of claim 10 wherein said spaced apart reflective surfaces are concave and arranged along an arcuate locus.
 12. Optical apparatus for an infrared intrusion detection system comprising; a sensing element for providing an electrical signal corresponding to the level of infrared radiation impinging on the element, and a plurality of spaced apart reflective surfaces for gathering infrared radiation from a plurality of discrete fields of view and focusing said radiation on said sensing element, at least some of said reflective surfaces having different focal lengths, adjacent reflective surfaces being of different focal length and disposed along different locii of curvature.
 13. The optical apparatus of claim 12 wherein the reflective surfaces include a center surface of first focal length and adjacent surfaces of progressively larger focal length.
 14. Optical apparatus for an infrared intrusion detection system comprising a sensing element for providing an electrical signal corresponding to the level of infrared radiation impinging on the element, and a plurality of spaced apart concave reflective surfaces arranged along an arcuate locus and for gathering radiation from a plurality of spaced apart reflective surfaces for gathering infrared radiation from a plurality of discrete fields of view and focusing said radiation on said sensing element, said sensing element being disposed at the focal point of only one of said reflective surfaces thereby being generally closer to said one reflective surface to more sharply focus radiation received from distant objects. 